Project Summary Prion diseases are infectious neurodegenerative diseases caused by the induced conformational change of a host-encoded glycoprotein, PrPC, into a pathogenic conformer, PrPSc. Currently, there are no vaccines or therapies available for these invariably fatal diseases, primarily because we do not understand the mechanism of PrPSc formation. Interestingly, there are large differences in host susceptibility to prion infection between different animal species. For instance, rabbits appear to be resistant to all prion strains, while bank voles appear to be a universal host. This variation, which is dependent on PrP sequence, provides us with a unique opportunity to identify the key steps in PrPSc formation shared by all species. Using rigorous biochemical tools developed in our laboratory, we will exploit these naturally occurring differences in host susceptibility to dissect the mechanism of prion replication. Specifically, this powerful comparative biochemistry approach will be used to accomplish the following aims: 1. Directly test the protein-only hypothesis of prion infectivity. 2. Identify and characterize PrPC domains that control susceptibility to prion conversion. 3. Determine whether cofactor molecules and post-translational modifications restrict the host range of prion infection. The results of this project will greatly advance our understanding of the prion replication mechanism. They will also impact our understanding of related, prion-like diseases, such as Alzheimer's and Parkinson's disease.